Memory devices are often used in electronic systems and computers to store information in the form of binary data. These memory devices may be characterized into various types, each type having associated with it various advantages and disadvantages.
For example, random access memory (“RAM”) which may be found in personal computers is typically volatile semiconductor memory; in other words, the stored data is lost if the power source is disconnected or removed. Dynamic RAM (“DRAM”) is particularly volatile in that it must be “refreshed” (i.e., recharged) every few microseconds in order to maintain the stored data. Static RAM (“SRAM”) will hold the data after one writing so long as the power source is maintained; once the power source is disconnected, however, the data is lost. Thus, in these volatile memory configurations, information is only retained so long as the power to the system is not turned off. In general, these RAM devices can take up significant chip area and therefore may be expensive to manufacture and consume relatively large amounts of energy for data storage. Accordingly, improved memory devices suitable for use in personal computers and the like are desirable.
Other storage devices such as magnetic storage devices (e.g., floppy disks, hard disks and magnetic tape) as well as other systems, such as optical disks, CD-RW and DVD-RW are non-volatile, have extremely high capacity, and can be rewritten many times. Unfortunately, these memory devices are physically large, are shock/vibration-sensitive, require expensive mechanical drives, and may consume relatively large amounts of power. These negative aspects make such memory devices non-ideal for low power portable applications such as lap-top and palm-top computers, personal digital assistants (“PDAs”), and the like.
Due, at least in part, to a rapidly growing numbers of compact, low-power portable computer systems in which stored information changes regularly, low energy read/write semiconductor memories have become increasingly desirable and widespread. Furthermore, because these portable systems often require data storage when the power is turned off, non-volatile storage device are desired for use in such systems.
One type of programmable semiconductor non-volatile memory device suitable for use in such systems is a programmable read-only memory (“PROM”) device. One type of PROM, a write-once read-many (“WORM”) device, uses an array of fusible links. Once programmed, the WORM device cannot be reprogrammed.
Other forms of PROM devices include erasable PROM (“EPROM”) and electrically erasable PROM (EEPROM) devices, which are alterable after an initial programming. EPROM devices generally require an erase step involving exposure to ultra violet light prior to programming the device. Thus, such devices are generally not well suited for use in portable electronic devices. EEPROM devices are generally easier to program, but suffer from other deficiencies. In particular, EEPROM devices are relatively complex, are relatively difficult to manufacture, and are relatively large. Furthermore, a circuit including EEPROM devices must withstand the high voltages necessary to program the device. Consequently, EEPROM cost per bit of memory capacity is extremely high compared with other means of data storage. Another disadvantage of EEPROM devices is that, although they can retain data without having the power source connected, they require relatively large amounts of power to program. This power drain can be considerable in a compact portable system powered by a battery.
In view of the various problems associated with conventional data storage devices described above, a relatively non-volatile, programmable device which is relatively simple and inexpensive to produce is desired. Furthermore, this memory technology should meet the requirements of the new generation of portable computer devices by operating at a relatively low voltage while providing high storage density and a low manufacturing cost.